This invention relates to guided weapon control systems and more particularly to a guidance steering law for such systems which implements proportional navigational guidance with high accuracy using a bank-to-turn autopilot.
Guided weapons have typically been of cruciform configuration having wing surfaces spaced 90.degree. apart. Although such a configuration is well known as are corresponding control methods, the configuration is not most advantageous from a packaging point of view. Especially when range requirements dictate that the wings be large, a cruciform configuration becomes quite unwieldy.
An alternative to the cruciform configuration is a two-wing configuration with both wings lying in a single plane. Such a configuration allows the wings to be made collapsible, so that they may be contained in a very compact convenient package. Whereas a cruciform configuration provides for control of the guided weapon trajectory in both pitch and yaw, a two-wing configuration provides control in only pitch, control in yaw being realized according to the so-called bank-to-turn (BTT) method.
In guided weapon control systems using a proportional navigational guidance (PNG) loop, pitch and yaw line-of-sight (LOS) rates from the guided weapon to a target are continuously measured and control steps taken in order to "null" the line of sight rates, or drive them to zero. The line-of-sight may be imagined as being represented by an elastic strand stretched between the guided weapon and the target. Relative motion between the guided weapon and the target that moves the elastic strand up, down, left or right produces a non-zero line-of-sight rate that generates a corrective control action proportional to that line-of-sight rate. Simple contraction of the elastic strand with diminishing distance between the guided weapon and the target represents that the guided weapon is on a collision course and does not generate any corrective control action. In the bank-to-turn method, the body coordinates of the guided weapon are rotated with respect to inertial coordinates by means of a roll maneuver such that the line-of-sight (LOS) rate with respect to the target lies entirely in pitch.
Typical bank-to-turn autopilots have been comparatively slow and imprecise. If roll and pitch maneuvers are executed simultaneously there will be some movement in the plane perpendicular to the desired plane. If the pitch maneuver is delayed until the roll maneuver is completed, the execution time of the overall maneuver is protracted.
Furthermore, typical bank-to-turn autopilots have been highly susceptible to the effects of tracking noise and glint. Glint refers to variation of the apparent radar center of a target due to angular motion causing differential phase changes at the receiver between the returns scattered from different parts of the target. Glint may be considered as a very low frequency source of noise originating at the target. Tracking noise, or "angular noise", refers to the effect of ever-present thermal noise as it is picked up and amplified by the angle channel receiver of a radar system. Tracking noise may be considered as a source of "white", or frequency independent, noise originating at the target.
An object of the present invention, therefore, is to achieve an improved bank-to-turn autopilot.
A further object of the present invention is to achieve a bank-to-turn autopilot that demonstrates high accuracy.
Still another object of the present invention is to achieve a bank-to-turn autopilot demonstrating high accuracy in the presence of tracking noise and glint.
Another object of the present invention is to achieve a bank-to-turn autopilot demonstrating relatively quick response.